Minor wording improvements to the contributor agreements' README

Clarified that the CLAs are Apache-based
Clarified that the "corporate" form applies to other organizations as well
Clarified that signing these doesn't give write permissions so much as commit
permissions.
Merge pull request #251 from bradcray/master

Minor wording improvements to the contributor agreements' README

* Clarified that the CLAs are Apache-based
* Clarified that the "corporate" form applies to other organizations as well
* Clarified that signing these doesn't give write permissions so much as commit permissions.